1. Field of the Invention
This invention relates to a dampening water control method and a printing apparatus for use in a lithographic printing that uses dampening water.
2. Description of the Related Art
In a lithographic printing that uses dampening water, the feed rate of dampening water is known to influence print quality. In actual practice, generally, the operator of the printing apparatus visually checks prints, and empirically determines a feed rate of dampening water. In one conventional technique, a film thickness of dampening water on the surface of a printing plate or a dampening water roller is measured, and control is carried out to maintain the film thickness constant.
In view of the above situation, Applicants herein have developed an apparatus for printing, along with a subject image, detecting patches that show density variations occurring with variations in dampening water, and controlling the feed rate of dampening water while measuring densities of the detecting patches (e.g. Japanese Unexamined Patent Publication No. 2002-355950).
The above prior apparatus is capable of automatically controlling the feed rate of dampening water by measuring the densities of the detecting patches. This provides an advantage of assuring a proper feed rate of dampening water without relying on the operator's experience as was the case previously. However, the above prior apparatus has drawbacks of requiring a relatively complicated computation, and providing only a small range of density variations of the detecting patches, to render the control difficult.